A dental prosthesis is one of the components which make up a dental implant which is a structure which is anchored to the jaw bone (maxilla or mandible) of a patient to replace one or more lost natural teeth. A dental implant comprises a dental prosthesis support structure which is anchored to the jaw bone of a totally or partially edentulous patient to support the dental prosthesis. The dental prosthesis support structure is typically formed by (i) one or more anchoring members (fixtures) implanted in the bone tissue of the maxilla or mandible, the number of fixtures used depending on whether the dental implant is to be for a single-tooth replacement, in which case a single fixture is used, or for replacement of a number of lost teeth, in which case one or, more usually, a plurality of fixtures will be used, and (ii) a transmucosal member (abutment) for each fixture, each abutment being connected to the coronal end of the associated fixture to bridge the soft tissue layer (gingiva) overlying the fixture site. A structure which presents one or more artificial replacement teeth--the dental prosthesis--is then connected to the coronal end(s) of the abutment(s), e.g. a crown for a single-tooth replacement or a bridge for replacement of a number of lost natural teeth.
To ensure that the dental prosthesis fits in with the prevailing conditions in the oral cavity, for instance the natural teeth still left in the maxilla or mandible, the dental prosthesis is fabricated with reference to a full-scale model of the toothless jaw prepared in a manner known per se. Usually incorporated in the model are one or more analogue components, the number corresponding to the number of fixtures implanted in the jaw bone. The or each analogue component has an end which is an analogue of the coronal end(s) presented by the dental prosthesis support structure, e.g. by the abutment(s). The or each analogue component is incorporated into the model so that the analogue end accurately reproduces the position and orientation that the associated coronal end of the dental prosthesis support structure has, or will have, in the jaw.
There is then provided for each analogue component a dental implant component which is commonly referred to in the art as a "cylinder" even though this component does not necessarily have a circular cross-section of constant diameter. Each such cylinder has a distal or apical end which complements the coronal end(s) of the dental prosthesis support structure and thus is able to seat on the analogue end of the associated analogue component. The cylinder(s) is seated on the analogue component(s) and then used in the fabrication of a dental prosthesis which fits in with the prevailing conditions in the oral cavity whereupon the dental prosthesis is transferred to the dental prosthesis support structure for anchorage to the jaw bone.
The manner in which the cylinder(s) is used in the fabrication of a dental prosthesis depends on whether the dental prosthesis is for a single-tooth replacement or for replacing a number of teeth and also on whether the prosthesis is to be a permanent prosthesis or a temporary prosthesis for use in the interim period while a permanent prosthesis is fabricated.
Permanent dental prostheses are typically formed by applying a wax pattern on the cylinder(s) and forming a cast structure on the cylinder(s) having the same profile as the wax pattern by the lost-wax casting process in which the cylinder-wax pattern assembly is surrounded by investment material and the wax pattern then removed by heating to leave a mould for a precious or semi-precious metal such as gold to be poured into via a sprue passing through the investment material. In this regard, the cylinder(s) can be totally or partially formed of a burnout material, e.g. of a plastic such as polystyrene, in which case the burnout material is also selectively replaced by the precious or semi-precious metal. The use of the investment material, however, ensures that surface features provided at the apical end(s) of the cylinder(s), for example the faces of a polygon, which enable the cylinder(s) to index with the coronal end(s) of the dental prosthesis support structure are retained in the cast structure when presented by burnout material.
For a permanent single-tooth dental prosthesis a porcelain layer will typically be fired onto the inner cast framework which includes, or is based on, a single cylinder to give a natural tooth appearance to the prosthesis. The wax pattern may or may not be shaped to give a tooth-like inner cast framework onto which the porcelain layer is fired. If the wax pattern is not shaped to a tooth-like form, the porcelain layer is built-up so as to give the prosthesis a tooth-like form.
Where a number of natural teeth are to be permanently replaced the dental prosthesis will typically comprise an inner cast framework which includes, or is based on, a number of cylinders and a range of artificial teeth (denture) mounted on the inner cast framework. In this instance, the wax pattern will typically splint the cylinders on the analogues in the model to form a cast framework which can bridge across the spaced-apart coronal ends of the dental prosthesis support structure and the denture will be mounted on the casting either before or after the casting is mounted on the dental prosthesis support structure. Alternately, the dental prosthesis may be formed by applying an isolated wax pattern to each cylinder to form individual castings which are then mounted to the coronal ends of the dental prosthesis support structure with the denture then being mounted onto the castings.
For an overview on the formation of permanent dental prostheses see inter alia `Dental Implants: A Guide for the General Practitioner`, Michael Norton, Quintessence Publishing, 1995, Chapters 4 and 5.
The use of temporary dental prostheses is well-known and the circumstances in which a temporary dental prosthesis is needed are documented in U.S. Pat. No. 5,259,759 (Jorneus et al/Nobelpharma AB). In the case of a temporary single-tooth dental prosthesis, the usual procedure is to form an acrylic resin artificial tooth on the cylinder by applying the acrylic resin to the outer surface of the cylinder when mounted on the analogue end of the analogue in the model, shaping the acrylic resin into a natural tooth form and then curing the acrylic resin.
The convention in the art is for a cylinder to be in the form of a sleeve, that is to say, having a distal or apical end for engagement with the analogue end of an analogue, and therefore the associated coronal end of the dental prosthesis support structure, a proximal or coronal end spaced from the apical end and an open-ended channel which extends from an opening in the apical end to an opening in the coronal end. Such cylinders are disclosed in U.S. Pat. No. 5,259,759 supra, U.S. Pat. No. 5,527,183 (O'Brien/Collaborative Enterprises, Inc.), U.S. Pat. No. 5,533,898 (Mena), U.S. Pat. No. 5,439,380 (Marlin), U.S. Pat. No. 5,180,303 (Hornberg et al/Regents of the University of California) and U.S. Pat. No. 5,234,339 (Grigereit/Kulzer, Inc.). In some of these U.S. patents the cylinders are fixed directly to a fixture, that is to say, the need for an abutment in the dental prosthesis support structure is obviated.
By using a sleeve cylinder in the formation of a dental prosthesis for the replacement of a plurality of lost teeth, e.g. a bridge, an inner cast framework having open-ended channels therethrough can be formed. In the case of a double construction bridge (see Chapter 5, `Dental Implants: A Guide for the General Practitioner` supra) these channels act as access passages for screws to pass through and secure the inner cast framework to the dental prosthesis support structure. The denture is then cemented to the inner cast framework screw-retained to the dental prosthesis support structure. In a regular bridge, the access passages can communicate with access passages formed in the denture when the denture is cemented to the inner cast framework. The cemented assembly can then be screw-retained to the dental prosthesis support structure via the communicating access passages.
In the case of a single-tooth dental prosthesis, e.g. a crown, the use of a sleeve cylinder allows the dental prosthesis to be cemented extra-orally to a coronal member of the dental prosthesis support structure such as an abutment with the cemented assembly then being added to the apical structure of the dental prosthesis support structure already anchored to the maxilla or mandible, as will be discussed more fully hereinafter.
In the case of permanent dental prostheses the formation of the access passages is as a result of a post being inserted into each sleeve cylinder when mounted on the associated analogue in the model to an operational position in which (i) the post is releasably secured to the analogue with the trailing end of the post projecting from the opening in the coronal end of the sleeve cylinder to a height greater than that to which the wax pattern is to be built, and (ii) the post forms a close fit with the opening in the coronal end of the sleeve cylinder. The wax pattern is then built-up around the post whereby removal of the post after the wax pattern has been completed leaves a passage in the wax pattern which communicates with the channel in the sleeve cylinder. The passage so formed in the wax pattern is then reproduced in the casting.
The close fit of the post with the opening in the coronal end of the sleeve cylinder acts to prevent wax leaking into the sleeve cylinder prior to casting which might otherwise make it difficult for the prosthesis or core part thereof to be properly secured on the dental prosthesis support structure.
A post is also used in similar fashion when a sleeve cylinder is used for forming a temporary dental prosthesis. For example, to form a temporary single-tooth dental prosthesis the apical end of the sleeve cylinder is engaged to the analogue end of the analogue structure in the model of the partially edentulous patient and the post is inserted into the sleeve cylinder to an operational position in which (i) the post is releasably secured to the analogue structure with the trailing end of the post projecting from the opening in the coronal end of the sleeve cylinder to a height greater than that to which the acrylic tooth is to be built, and (ii) the post forms a close fit with the opening in the coronal end of the sleeve cylinder. Acrylic resin is then applied to the outer surface of the sleeve cylinder around the post, shaped into the form of a natural tooth which inter alia fits in with the surrounding dentition in the model and then cured.
The close fit of the post with the opening in the coronal end of the sleeve cylinder in this instance acts to prevent the acrylic resin leaking into the sleeve cylinder prior to curing which might otherwise make it difficult for the prosthesis to be properly secured on the dental prosthesis support structure. The post further acts to form a channel in the cured acrylic tooth which communicates with the channel in the sleeve cylinder or, in other words, to form an open-ended channel through the temporary single-tooth dental prosthesis through which, for example, a screw can pass to screw-retain the prosthesis to the dental prosthesis support structure.
To illustrate the fact that the use of a sleeve cylinder enables extra-oral connection of a single-tooth dental prosthesis to a coronal member of a dental prosthesis support structure, as mentioned briefly hereinabove, one can consider the case of securing a temporary single-tooth dental prosthesis of the type described to a dental prosthesis support structure having a coronal member which is a conventional abutment comprising (i) an abutment sleeve having an open-ended axial channel which sits on the coronal end of a fixture in which there is an internally-threaded socket, and (ii) an abutment screw which is inserted through the channel in the abutment sleeve to be screwed into the internally-threaded socket in the coronal end of the fixture to secure the abutment sleeve to the fixture through a head of the abutment screw bearing down on a shoulder in the channel of the abutment sleeve. The sleeve cylinder sub-structure of the temporary single-tooth dental prosthesis will be cemented to the sleeve of this two-part abutment. In this connection, the open-ended channel formed in the temporary single-tooth dental prosthesis by the post enables the sleeve cylinder to be cemented to the abutment sleeve extra-orally and the assembly so formed to be placed on the coronal end of the fixture with the abutment screw then being inserted through the prosthesis-abutment sleeve assembly for screw engagement in the internally-threaded socket in the fixture. The channel in the prosthesis would then be filled in with a cosmetic material.
In the above illustration of extra-oral connection it is conceivable that the abutment sleeve could be releasably secured in the model in place of an analogue thereof, for example on an analogue of the fixture, whereby the sleeve cylinder is placed on the abutment sleeve during the fabrication of the dental prosthesis. In this case, the post could be secured in an internally threaded socket in the fixture analogue or, where the abutment screw is used to screw retain the abutment sleeve to the fixture analogue, in the abutment screw itself. More generally, there are other conceivable instances in which the coronal members of a dental prosthesis support structure may be incorporated in a model for use in combination with a sleeve cylinder during fabrication of a dental prosthesis prior to being anchored to the jaw bone.
Hitherto proposed access passage-forming posts have been releasably securable to the support component for the sleeve cylinder in the model by providing the posts with a screw threaded distal end for screw retention in an internally-threaded socket in the support component. An aim of the present invention is to provide a simpler connection between the access passage-forming post and the support component for the sleeve cylinder.